Where do I use Iron Coat hero?

Iron coat: Cheng Zheng is a purple combat hero. You assign him to one of the legions. If you ave already unlocked all of the skills then the best placement for him is the front row. But this only affects skill 5. In general most players will work on recruiting and and upgrading the orange heroes and purple heroes are just fillers for legion slots until you have enough so it's not important where to place it.